Adding a State of Charge Monitor

Dec 2013: Most of the description here is out of date with the fitting of a
lithium battery. It is retained for historical interest.

Since the fridge is on-line in the Applause all the time, measuring the
at-rest battery voltage to estimate the energy state of
the house battery is not really possible. A State of Charge monitor measures the current to
the battery and integrates this over time to give a running total in terms of Amp-hours as
the battery is charged and discharged. This gives a far more accurate picture of
energy stored in the battery.
Expressed as a percentage, a full battery reads 100%; we try not to go below 70%,
and if the battery state of charge goes below 50%, damage will have been done.

Available SOC Monitors

Several of the more expensive Solar Power Managers
(e.g.,
Plasmatronics) include a State of Charge or Energy State function.

Silicon Chip described
a "Battery Capacity Meter Circuit" in the June 1998 issue. No kit was ever made available,
and the circuit used now out of date components and was unduely cumbersome though it was, for
its time, quite advanced in using a PIC programmable microcontroller monitoring the amplified
voltage drop across a current shunt.

It seems that Hall-effect sensors have now
developed to the point where they are stable enough to use routinely to measure current
in challenging conditions. The LEM
LTS 25-NP is one such, and it is combined with a PICAXE in a
kit from MTM Scientific, Inc. It is this
US$59.00 kit that I have built and installed in the Applause.

The MTM Scientific Battery Amp-Hour Meter

The supplied kit consists of all components and a double-sided circuit board and went together
easily. A ten-turn potentiometer is used to set the battery A-hr capacity, which is displayed at
power-up for 60 s. I found that at first the display counted up rapidly, uncontrollably. Adjusting
the potentiometer over a broad range made no difference. It was only when I set the potentiometer
fully anticlockwise that the circuit performed correctly. I was then able to set it to the correct
battery capacity (The present battery rating is 120 A-hr). After a minute, the display shows
percentage capacity remaining, between 0 and 100%. Current consumption is 40 mA.

For the present application the Hall-effect sensor is mounted remotely, close to the battery,
and the main circuit board is mounted on the wall behind the battery, on the wall shared with the
gas bottle compartment. The LTS 25-NP sensor has a 3 mm diameter hole through which the current
carrying wire passes. I use a short length of 10 AWG (4.9 mm²) wire for this task.
A four-colour ribbon cable joins the sensor and the main board, and 0.1 µF noise suppression
capacitors are mounted on the sensor sub-board: between the +12 V supply for the main board and
ground, and between the +5 V supply to the sensor and ground.

The diagram shows the changes I have made to the wiring to the house battery to include
the LTS 25-NP Hall-effect sensor. At the same time I also included a new 100 mV/100 A current shunt
(from The 12 V Shop, though I have since
discovered that Jaycar sell an inferior product at a third the price). Photographs show the
components on their mount board, the board with the covers on, and the Amp-Hour Meter in
position.

Monitoring State of Charge

Starting up: as stated in the instructions, it is important that no current flows through
the current sensor during the first minute of operation. The PICAXE uses the first minute to
determine what sensor output voltage corresponds to zero current flow; if there is any current it
will be interpreted as a voltage offset, and later current flows will be erroneously measured. This
means that nothing else can be connected to the battery when the Amp-Hour Meter is started - a
nuisance that is easily dealt with by powering the meter from the second sense point on
the current shunt. Then I can simply disconnect and connect the terminal block to the shunt.

In service: Since the meter will not read over 100% battery capacity, there is no need
to be concerned about the initial reading of battery state - it will by either the mains charger
or the solar panels, at some stage reach 100% charge. From then on the meter should read the
present state of charge, resetting itself each time 100% charge is achieved. So long as the
100% stage occurs for long enough then the battery will truely reach full charge. (Since internal
resistances must be overcome, full charge of the battery will actually take longer to achieve,
perhaps 10% longer than when the meter says.)

Along with the house battery, the meter is located under the foot of the bed at the back
kerbside. Its readings can be viewed by raising the bed, and opening the hatch to the battery
compartment. More convenient is to read the meter from the the roadside rear access door to
the hot water service and solar panel regulator, where the performance of the solar panels can
also be consulted. This is achieved by locating two mirrors so they reflect the meter display.
The diagram makes this clearer, and some photos show what can be seen from that door.

The lowest State of Charge observed so far is 64% — i.e., Depth of Discharge 36%.

Reliability of State of Charge Reading

The PICAXE algorithm is fairly simple. As already mentioned, it does not recognise that
charging the battery requires more energy than can be withdrawn due to losses — charging
losses of more than 10% are common. Nor does the algorithm attempt to compensate for charge and
discharge temperatures, which affect the energy that can be withdrawn. Nor that the energy
available from a battery varies according to how rapidly it is discharged (and charged) —
Peukert's Law. Nor that the battery
capacity reduces with age.

Very few State of Charge meters account for all of these effects. In normal operation (i.e., not
when the whole system is at idle for days or more) we can mitigate most of these issues:

ensure that readings are only considered if indicated full charge has been maintained for
at least an hour or so within the last 2–3 days;

the battery is within the house, and temperatures there are unlikely to vary greatly due to
human comfort factors;

The rated capacity of the
Fullriver HGL battery, the C20 value, is 6 Amp for 20 hours, the battery is near-new,
self-discharge is very small at 0.7% per week, and the
Peukert exponent for
normal current draw in the Applause (1–20 Amp) is particularly low at 1.133.

In practice in the Applause the current draw is mostly below the C20 value of 6 Amp, so the Peukert
effect error is likely to be quite small (it would be zero for 6 Amp draw). Combinations of fridge
and water pump or vacuum toilet pump are active for only seconds at a time. But with TV and lights
on for an hour or more the current can be 7–8 Amp for the time the fridge is on, reaching
15–20 Amp when the water pump is also on. I expect the error will to lead to a slight
underestimate of the indicated State of Charge: a useful safety effect.